Referring to FIG. 1, a gas turbine engine is generally indicated at 10 and comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high pressure compressor 14, a combustor 15, a turbine arrangement comprising a high pressure turbine 16, an intermediate pressure turbine 17 and a low pressure turbine 18, and an exhaust nozzle 19.
The gas turbine engine 10 operates in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 which produce two air flows: a first air flow into the intermediate pressure compressor 13 and a second air flow which provides propulsive thrust. The intermediate pressure compressor compresses the air flow directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high pressure compressor 14 is directed into the combustor 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive, the high, intermediate and low pressure turbines 16, 17 and 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low pressure turbines 16, 17 and 18 respectively drive the high and intermediate pressure compressors 14 and 13 and the fan 12 by suitable interconnecting shafts 26, 28, 30.
It will be understood that in order to accommodate for the possible occurrence of blade fragmentation the casing parts of the engine must be able to restrain blade and other debris. In such circumstances these casing parts are typically thicker and specified to provide a degree of surety with respect to such blade fragment containment. One approach is to reduce the energy transfer to the casing by encouraging controlled break up of the blade when inevitable and in particular of the blade root fragments. It will be understood as blades become lighter through having a hollow construction or composite construction the root section increasingly incorporates a larger percentage of blade mass.
As indicated above it is known to encourage break up of blade fragments in order to reduce localised impact energy transfer and therefore casing requirements. One approach to encouraging such break up is through introducing lines of weakness in the form of a break line. These lines of weakness are drilled or otherwise machined into the blade. A disadvantage of such an approach is that the blade and in particular a composite blade will include a moisture path in the line of weakness which may then precipitate cracking through freeze-thaw cycles. It will be understood that premature cracking will result in a shorter operational life for the blade and therefore increase costs for maintenance as well as replacement. Further problems with respect to lines of weakness in a blade are the potential for tool breakage and damage to the blade which as will be understood at this stage is a high value component. Finally, provision of cavities and other lines of weakness can be difficult to model in terms of responsiveness and add significantly to potential problems with stress in normal operational conditions for the blade.
FIG. 2 provides a schematic illustration of a root fragment 30 in cross section impacting a casing 31. Thus, it will be noted that parts of the fragment 30 engage and react at points 32, 33 with the casing 31 whilst there is a large bending moment 34. The fragment 30 distorts as a result of momentum with a fragment velocity in the direction of arrowhead 35. In such circumstances in a portion 36 of the fragment 30 there will be an increased nominal stress due to the reduced load bearing area along with an increased compliance of the root fragment 30. This results in plasticity and cracking which dissipates energy. Thus, there is a plastic, non-elastic hinge defined about the portion 36 which allows the fragment 30 to flatten and so increase contact area with the casing 31. In cases where the root is curved (as opposed to being straight) the point contact forces are inevitable, as shown in FIG. 2. In such circumstances the larger the fragment 30 the greater the energy of impact with the casing 31 and therefore potential breach. In such circumstances the casings 31 need to be relatively thick and therefore add significantly to overall weight requirements for an engine in an aircraft. As will be appreciated weight is a significant design consideration with respect to aircraft.